Petroleomics: Chemistry of the underworld Alan G. Marshalla,b,1 and Ryan P. Rodgersa,b,1 aNational High Magnetic Field Laboratory, Florida State University, 1800 East Paul Dirac Drive, Tallahassee, FL 32310-4005; and bDepartment of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306 Edited by Fred W. McLafferty, Cornell University, Ithaca, NY, and approved August 14, 2008 (received for review May 24, 2008) Each different molecular elemental composition—e.g., CcHhNnOoSs— neutrals. Because many heteroatom-containing components has a different exact mass. With sufficiently high mass resolving (NnOoSs) of petroleum are highly polar, ESI is specific and power (m/⌬m50% Ϸ 400,000, in which m is molecular mass and ⌬m50% especially efficient in generating their gas-phase ions. Although is the mass spectral peak width at half-maximum peak height) and petroleum crude oils typically contain 90% hydrocarbons mass accuracy (<300 ppb) up to Ϸ800 Da, now routinely available (CcHh), the NnOoSs-containing molecules are typically the most from high-field (>9.4 T) Fourier transform ion cyclotron resonance problematic with respect to pollution, fouling of catalysts, for- mass spectrometry, it is possible to resolve and identify uniquely and mation of deposits during production and processing, corrosion, simultaneously each of the thousands of elemental compositions emulsions, and the highest-boiling fractions of lowest economic from the most complex natural organic mixtures, including petroleum value. ESI coupled with low-resolution MS was first applied to crude oil. It is thus possible to separate and sort petroleum compo- petroleum by Zhan and Fenn (6). High-resolution ESI FT-ICR nents according to their heteroatom class (NnOoSs), double bond MS of petroleum has subsequently resolved and identified number of rings plus double bonds involving Ͼ17,000 different elemental compositions for organic bases and ؍ equivalents (DBE carbon, because each ring or double bond results in a loss of two acid in crude oil (7). hydrogen atoms), and carbon number. ‘‘Petroleomics’’ is the charac- Access to many of the remaining 90% of petroleum compo- terization of petroleum at the molecular level. From sufficiently nents is afforded by field desorption/ionization (FD) and atmo- complete characterization of the organic composition of petroleum spheric pressure photoionization (APPI). Continuous-flow FD and its products, it should be possible to correlate (and ultimately FT-ICR MS yields abundant ions from several species not predict) their properties and behavior. Examples include molecular observed by ESI, including benzo- and dibenzothiophenes, mass distribution, distillation profile, characterization of specific frac- furans, cycloalkanes, and polycyclic aromatic hydrocarbons tions without prior extraction or wet chemical separation from the (PAHs) (8). However, FD experiments are slow because of the original bulk material, biodegradation, maturity, water solubility (and need to ramp the current to the FD emitter over a period of a oil:water emulsion behavior), deposits in oil wells and refineries, couple of minutes so as to volatilize/ionize species of successively efficiency and specificity of catalytic hydroprocessing, ‘‘heavy ends’’ increasing boiling point. APPI FT-ICR MS (9) can accumulate (asphaltenes) analysis, corrosion, etc. one mass spectral dataset in a few seconds and is thus better suited for signal averaging of a few hundred scans for increased Fourier transform ͉ ion cyclotron resonance ͉ mass spectrometry ͉ dynamic range. APPI of petroleum requires the ultrahigh reso- petroleum ͉ fossil fuel lution of FT-ICR MS for two reasons: (i) APPI ionizes a broader range of compound classes, and thus the mass spectrum contains he rapidly ballooning interest in characterization of petro- approximately five times as many peaks as an ESI mass spectrum of the same crude oil sample; and (ii) the same analyte molecule Tleum crude oil and its products derives from the confluence ϩ• ϩ ϩ of three recent developments: (i) the rapidly increasing cost of may produce both M and (M H) ions, making it necessary to resolve Mϩ• containing one 13C from (MϩH)ϩ containing all crude oil (up to $120 per barrel at this writing), (ii) the global 12 market shift toward heavier/more acidic/higher sulfur crude oil C, a mass difference of only 4.5 mDa (see below). Laser as the supplies of light ‘‘sweet’’ (low sulfur) crudes are depleted, desorption/ionization mass spectrometry, although highly useful and (iii) the introduction of ultrahigh-resolution mass analysis to for biomolecule analysis, does not provide a reliable represen- separate and identify up to tens of thousands of crude oil tation of petroleum components because of significant aggre- components in a single step. Because the organic composition of gation and fragmentation at the laser power required to generate petroleum is so complex, its characterization was until recently a useful number of gas-phase ions (10). However, recent intro- limited to bulk properties (e.g., density, viscosity, osmotic pres- duction of a two-color laser method (11), in which the first laser sure, light scattering, UV-visible and infrared spectroscopy, desorbs the neutrals and the second laser ionizes them, appears NMR, x-ray scattering, and absorption-edge spectroscopy) and to overcome the disadvantages of single-color laser desorption/ various wet chemical separations based on (e.g.) solubility, ionization. boiling point, gas chromatography, and liquid chromatography. Saturated hydrocarbons are especially problematic, because The historical development of those applications, as well as prior virtually all methods for ionizing neutral saturated hydrocarbons low- and high-resolution mass spectrometry of petroleum, have produce extensive fragmentation, thereby making it hard to been reviewed elsewhere (1). identify the neutral precursors in the original sample as well as Fourier transform ion cyclotron resonance mass spectrometry vitiating quantitation. Recently, laser-induced acoustic desorp- tion (LIAD) of neutrals, followed by chemical ionization with (FT-ICR MS) offers the highest available broadband mass appropriate reagents, has shown great promise in achieving resolution, mass resolving power, and mass accuracy (2). It was first applied to petroleum via electron ionization of petroleum distillates (3, 4). However, the relatively low magnetic field (3.0 Author contributions: A.G.M. and R.P.R. designed research; R.P.R. performed research; T) limited the m/z range, the need to volatilize the sample limited R.P.R. contributed new reagents/analytic tools; R.P.R. analyzed data; and A.G.M. wrote the the molecular weight range (especially for heteroatom- paper. containing species), and electron ionization produced extensive The authors declare no conflict of interest. fragmentation (especially of alkyl chains). This article is a PNAS Direct Submission. Electrospray ionization (ESI) (5) is most efficient for polar 1To whom correspondence may be addressed. E-mail: [email protected] or molecules and typically generates positive ions by protonating [email protected]. (basic) neutrals and negative ions by deprotonating (acidic) © 2008 by The National Academy of Sciences of the USA 18090–18095 ͉ PNAS ͉ November 25, 2008 ͉ vol. 105 ͉ no. 47 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0805069105 Downloaded by guest on September 29, 2021 petroleum mass spectrum (15). Ultrahigh mass accuracy is needed for a different reason. Fig. 1 shows the mass defects (defined in the figure) for the most abundant isotope of each of SPECIAL FEATURE several common chemical elements. Every isotope of every element has a different mass defect. (By definition, the mass defect of 12C is zero.) Thus, if the mass of a molecule can be measured with sufficiently high accuracy (in practice, Ϸ0.0003 Da for molecules up to Ϸ1,000 Da in mass), then its elemental composition can usually be determined uniquely from its mass alone (16, 17). Fig. 2 provides examples of some of the closest mass ‘‘splits’’ encountered in mass spectrometry of petroleum. Two of the most important close doublets are molecules whose 12 32 elemental compositions differ by C3 vs. SH4 (to identify sulfur-containing components, see Fig. 2) and 13C vs. 12C1H (for APPI MS, see above). Results and Discussion Molecular Mass Distribution. The first step in characterization of organics in petroleum is to determine the molecular mass distribution, particularly for ‘‘heavy ends’’ high-boiling compo- nents and residues. It is now well established that the average Fig. 1. Atomic mass defects for selected isotopes of some common chemical Ͻ elements. Because no two have the same mass defect, it is possible to deter- molecular mass of asphaltenes is 1,000 Da (18). Higher average mine a unique elemental composition for any molecule from a sufficiently molecular masses inferred from prior vapor-phase osmometry accurate mass measurement. and size-exclusion chromatography may be attributed to forma- tion of noncovalent aggregates because of too-high concen- tration and/or use of solvents that promote aggregation. Aggre- efficient (and uniformly efficient) ionization of saturated hy- gation has been demonstrated by mass-selective isolation of drocarbons over a wide molecular mass range (12, 13). singly charged dimer ions in the range 840–860 Da, followed by CHEMISTRY ‘‘Petroleomics’’ is the principle that from sufficiently complete collisional activation at collision energies too low to break characterization